In a solid-state laser, laser diodes function as efficient means for exciting solid-state laser medium. This is due to the fact that a wavelength of the light emitted from the laser diodes may be selected to match an absorption wavelength of the laser medium of interest. This implies that parasitic absorption of light, i.e. absorption not participating laser oscillation, is very little, so that little thermal expansion of the medium will take place, resulting in only negligible "lense effect" which causes local convergence of light. Thus, laser diode pumped solid-state lasers may easily carry out emission of a single mode transverse light waves. A further advantage of the laser diode pumped solid-state lasers is that they may be manufactured in compact and light forms, and have long life.
Presently, however, laser diode pumped solid-state lasers have the following difficulties. They are that: (1) the pumping power of the light source, i.e. laser diodes, is not high for optical pumping of a solid laser medium, and (2) absorption coefficient of laser medium is small, obstructing the realization of a high power laser. The second problem (2) arises because active ions such as Nd.sup.3+ that can participate in laser light emission may be doped in a host medium to only limited concentration. This is also the case when YAG crystal is used as a host medium. To overcome these problems, efforts have been directed mainly to develop: (A) a laser medium having a high optical absorption coefficient and a high pumping efficiency, and (B) an excitation scheme for efficient pumping of the medium by laser diodes. In pursuit of a solution to the first object (A) above, effort has been made mainly to find a best arrangement of a multiplicity of laser diodes for optimum optical pumping. For the second object (B), improvements made in the past are based on provision of a longest optical path possible for the light beam to be absorbed by the medium.
FIG. 1 illustrate a prior art laser developed to overcome some of these disadvantages. In this arrangement a multiplicity of laser diodes 1 provide pumping light, which is collected at a focus of a lense 3 by a bundle of optical fibers 2, which light is in turn collected on a laser medium 5 by a lense 4. An optical resonator comprises a partially transmitting mirror 6 and a totally reflecting mirror 7 at the laser oscillating wave length. Mirror 7 is transparent for the pumping light from the laser diodes so that the incident pumping light from the laser diodes penetrates into the medium 5. Lasers of this type are called end-pumped solid-state lasers because they perform optical pumping with light entered the medium at the opposite end face of the medium. Advantageously, they have a pumping region which coincides with a region where oscillatory emission modes are established, so that the clear problem of lens effect mentioned above and may provide a high power single transverse mode. Since the incident light enters the medium from its longitudinal end, they also attain the second object (B).
FIG. 2 illustrate another prior art laser, in which beams of pumping light emitted from "a diode bar" 11, which is a multiplicity of laser diodes, are collimated by a fiber lense 12 onto a long side face of a laser medium 13. The laser medium is a Nd.sup.3+ YAG crystal having dimensions of, for example, 3.times.5.times.20 mm. The face of the medium receiving the incident light is provided with a coat which may totally reflect YAG laser light whose wavelength is 1.06 microns, but is transparent for the pumping light whose wavelength is about 0.8 microns. Mirrors 14 and 15 together constitute a resonator. Lasers of this type are called side-pumped solid-state lasers. Since they have a long zig-zag optical path 16 over the entire length of the medium 13 excited by the multiple diode lasers, they may provide high output laser power.
As discussed above, either type of prior art lasers have some improvements on output power through improved designs in configuration and dimension of laser medium, and improved arrangements of pumping laser diodes. However, in the case of Nd.sup.3+ YAG crystal laser, maximum laser output power rate (i.e. output power per unit volume of laser medium) is limited by Nd.sup.3+ concentration in the YAG crystal. Maximum concentration of Nd.sup.3+ doped in the YAG crystal is at most a few per cent. The maximum power rate cannot be improved by any excitation scheme. Although it has been reported recently that LiNdP.sub.4 O.sub.14 can be better host for Nd.sup.3+ at higher concentration thereof, the maximum laser output power rate is still limited by the concentration of the Nd.sup.3+ ions.